(In my study and experience) "B/C" means "a Bmaj triad, with a C note in the bass" - it does not mean "a Bmaj triad with a Cmaj triad in the bass". This is a short-hand method of indicating a chord voicing/sound that the composer wants to occur - It is not intended as an analysis of the chord sound. The composer is assuming that the players will have a deeper knowledge and an aural idea of how to play over this sonority.
Below is an example of that sonority - @ 0:21 - 0:26 - in this case it is Eb/E (an Ebmaj triad, with an E note in the bass.)
A "Polychord" is written with a HORIZONTAL line between the notes ( I can't type that format here) if you wanted to specify "a Cmaj triad and a Gbmaj triad to be played simultaneously" (Stravinsky's "Petrushka Chord", for example.)
My $0.03.
Thanks.